Just in from the Ship

TC-99-07 Lobster Assessment
June 6-July 4, 1999

     June 17, 1999    We are continuing with sampling the Necker Island reef lobster population (right) using lobster traps that are hauled aboard in the morning and set out in the afternoon for an overnight soak. During the night we perform ADCP transects which involves running the ship off the reef in a straight line for 40 miles or so, then turning around (usually around midnight) and running back along the same path to where the traps are soaking.
     The ADCP (Acoustic Doppler Current Profiler) (below left), works much like a depth sounder. Installed in the bottom of the ship, the unit transmits loud "pings" (sound) to the ocean below. The unit then listens for the sound to bounce back and it records the time it took and the direction from which the echo returned. Depth sounders record the sound bouncing off the sea bottom. The ADCP however, records the sound that bounces back from small particles floating in the water.
     The ADCP determines the speed at which the particles are moving by the way the reflected sound is altered. For instance, when a train (or car) passes you blowing its horn, the sound increases in pitch as the horn approaches and than decreases in pitch as the horn passes you by. This change in apparent sound is known as Doppler Shift, and if you practiced, you might get fairly good at estimating the speed of the train just by how fast the whistle is increasing or decreasing in pitch.
     This is similar to how the ADCP determines the current under the ship. It listens for how much and how fast the sound is altered as it reflects off the particles floating in the current. It does this simultaneously for many different depths to a maximum depth of around 200 meters (about 600 feet). We don't need to stop the ship either as the system automatically subtracts the ship's course and speed from the results.
     Understanding the ocean's currents is very important as most animals in the ocean are dependent on them in some aspect. Marine animals use currents to transport themselves (such as jelly fish, phytoplankton and zooplankton), bring them food (as in anemones and sponges), and disperse their young (lobsters, swordfish, tunas, marlins, and many reef fish).

Yesterdays cruise write-ups for TC-99-07